We unfortunately don't have the expertise to identify the species by a wood sample alone. You could check with a good lumber yard or tree harvester. Black walnut is very common in Michigan, so I wouldn't be at all surprised if that is what you have. Besides the natural genetic diversity, wood hardness and density are influenced to some degree by environmental factors and by moisture content of the wood. This means you will find some planks of the same species that are noticeably harder and denser than others.